In recent years, regulations on vehicle height have been relaxed as more emphasis has been placed on running stability and vehicle design, regardless of the type of vehicle—minivan, sports utility vehicle, and the like. As a result, there has been an increasing number of low-floor vehicles (vehicles with low ground clearance). While the low-floor vehicles have contributed to improving running stability and vehicle design mentioned above, they sometimes require adjustment of height when running onto a sidewalk (stepped portion) or coming down from a sidewalk to a road, for example.
A conventional height adjustable suspension unit, produced in response to the need, is now described with reference to FIG. 12. The conventional height adjustable suspension unit includes: a shock absorber S having a cylinder 50 connected to an axle (not shown) and an extensible rod 51 disposed in the cylinder 50 to be able to advance and retract, the extensible rod 51 having at its end a screw portion 52, which passes through a chassis X and to which a nut N is screwed and fixed; a coil spring 55 mounted around the extensible rod 51, the coil spring 55 having a lower end supported on an annular spring base 56 that is attached to the cylinder 50; a cylinder 60 through which the extensible rod 51 passes in the axial direction, the cylinder 60 being fixed to the chassis X; and a plunger 70 inserted into a fluid pressure chamber 80 of the cylinder 60, the plunger 70 being movable along the extensible rod 51 in the axial direction while being in contact with the coil spring 55.
The cylinder 60 has an annular contact portion 61 that is in contact with the chassis X and with the outer circumferential surface of the extensible rod 51; and a tubular covering portion 62 for covering the plunger 70, the tubular covering portion 62 being suspended from the periphery of the annular contact portion 61. The contact portion 61 has an annular groove 61a in its inner circumferential surface, and a packing P is fitted in the groove 61a to ensure sealing between the contact portion 61 and the extensible rod 51.
The plunger 70 includes: an annular rod-side sliding portion 71 that is slidable along the extensible rod 51; a tubular covering portion 72 for covering the outside of the coil spring 55, the tubular covering portion 72 being suspended from the periphery of the annular rod-side sliding portion 71; an annular cylinder-side sliding portion 73 that is slidable along the inner circumferential surface of a lower end opening 63 of the cylinder 60; and a flange-shaped stopper 74 for regulating upward movement of the cylinder 60 by coming into contact with the opening end surface of the cylinder 60, the stopper 74 being disposed on the outer circumferential surface of the lower end of the cylinder-side sliding portion 73 to project radially outward. The annular rod-side sliding portion 71 of the plunger 70, similarly to the contact portion 61 of the cylinder 60, has an annular groove 71a, and a packing P is fitted in the groove 71a. Thus, the plunger 70 is slidable along the extensible rod 51 while at the same time sufficient sealing is ensured. The cylinder-side sliding portion 73 also has a similar packing P on its outer peripheral surface. Thus, the plunger 70 is slidable along the inner circumferential surface of the cylinder 60 while at the same time sufficient sealing is ensured therebetween.
The fluid pressure chamber 80, through which fluid flows in and out, is formed between the inner surface of the cylinder 60 and the outer surface of the plunger 70. The fluid pressure chamber 80 includes: an annular upper space 80a formed between the contact portion 61 of the cylinder 60 and the rod-side sliding portion 71; and a tubular lateral side space 80b formed between the covering portion 62 of the cylinder 60 and the covering portion 72 of the plunger 70.
The cylinder 60 is provided with an attaching portion for an air valve (not shown), through which air is introduced or discharged to and from the fluid pressure chamber 80, and a certain valve is attached to the air valve attaching portion. An example of an air supply and discharge device for use includes an air tank and a compressor connected to the air tank. Such an air supply and discharge device is mounted on the chassis X to introduce air into a vehicle height adjusting unit from the air tank by the operation of a switch provided in the driver seat.
Supply of air is carried out according to a driver's determination. A driver can adjust vehicle height by appropriately adjusting the amount of air supply, not only when driving on roads of different levels, but also when driving on a rough road with bumps or driving on a flat asphalt paved road.
Described here is a manner in which the conventional height adjustable suspension unit is used. FIG. 12 illustrates a state in which: the fluid pressure chamber 80 has not been supplied with fluid (air); the stopper 74 of the plunger 70 is in contact with the opening end surface of the cylinder 60; and the extensible rod 51 of the cylinder 50 is retained at a low ground clearance position by the elastic force of the coil spring 55.
When height adjustment is needed to run onto a sidewalk (stepped portion), fluid (air) is supplied to the fluid pressure chamber 80 in the state of FIG. 12. Then, the air supplied to the fluid pressure chamber 80 presses the flat end surface of the rod-side sliding portion 71 and the annular flat surface of the cylinder-side sliding portion 73 of the plunger 70, as shown in FIG. 13, thereby pushing the plunger 70 downward. In this process, the coil spring 55 is moved downward with its length unchanged, without being compressed and deformed by the air, and the cylinder 50 is moved downward proportionately with the movement of the plunger 70. As a result, the shock absorber extends out, and the chassis X which has been kept at a low ground clearance position is lifted, so as to allow the vehicle to run over a stepped portion without causing any damage to the chassis X.
As another example of a conventional height adjustable suspension unit, one known suspension unit includes: a drive gear disposed concentrically about an end of a rod attached to a chassis; a tubular male screw mounted concentrically about the rod to mesh with the drive gear; and a tubular female screw that is threadedly engaged with the male screw, the tubular female screw being suspended from a spring receiving member disposed at the upper end of a coil spring.
With this height adjustable suspension unit, when the drive gear is rotated, the male screw starts turning while the female screw remains fixed. Thus, vehicle height can be adjusted by the movement of the male screw toward and away from the female screw.
Prior Art Document
Patent Document
Patent Document 1: Japanese Patent Application Laid-open No. Hei-11-108100